If there may be no risk in using Apparmor misconfigured, it’s still useless and not a good practice to use apps this way. Overall, this was more a general advice because other apps may have more negative effects when not properly configured. It’s good to set up good habits from the start and stick to them.

I’ve built my answer based on the use-case you exposed (i.e. not using untrusted packages). As I said, if you are now planning on using untrusted packages, you should configure your system appropriately.

Re: AUR, it’s quite safe. In theory it can be harmful but only if the user is not careful. You should always inspect PKGBUILDs and *.install files when building packages from the AUR (the pacman wrapper you use to download from AUR should have a dialogue which prompts you to do this). I have personally never experienced any troubles using packages from AUR (and I have quite some) because the community is usually pretty vigilant but also because I use only packages maintained for long time by known developers.

Hope this helps. You can learn more by reading the arch wiki (which is recognized to be the best one so use it at your advantage) and by doing simple searches on the arch forum. Both resources were dramatically helpful to me understanding what arch is, how it works, and what to expect when I started using it 15 years ago.